A 43-metre exhaust stack weighing more than 50 tonnes has crashed to the ground after slipping from two cranes at the Inpex-operated LNG project at Blaydin Point near Darwin.

NT WorkSafe is investigating. No-one was injured.

Unions NT secretary Paul Kirby said workers told him the chimney was almost vertical when it fell and it would have created a "pretty loud noise" when it crashed to earth.

However, an Inpex spokeswoman said the crane fell from "a short distance above the ground".

"The stack dropped the short distance and fell over its pre-lift position," the spokeswoman said.

"Work in the area has ceased."

Staff were alerted to the accident with a letter sent on Thursday night:

During repositioning of the heating stack the load become detached from the rigging assembly at the top lifting points resulting in damage as it fell to ground within the exclusion zone.

The alert also banned photography and ordered lifts greater than 50-tonnes be suspended.

Security are patrolling the area and photography's is not permitted of the scene and this will be enforced.

Mr Kirby complained Inpex took too long to alert staff and suspend activity.

"My understanding is that, for a good few hours, there were still large cranes working on that site," he said.

"It's just annoying for us that with lightning responses and things like that, information can get across the site within a matter of minutes, but with significant safety incidents we can't seem to get information across the site in a reasonable amount of time."

He said he believed the crane had been almost vertical when it detached from the rigging and fell.

"It would have fallen quite some significant distance and would have made a pretty loud noise when it hit, I imagine," he said.

The Japanese company Inpex is building LNG trains at Blaydin Point to process natural gas piped 889 kilometres from Browse Basin off the coast of Western Australia.

The facility is part of the $US34 billion Ichthys project, which has contributed to a major resources and employment boom in the Northern Territory.

WorkSafe issued a statement confirming an "exclusion zone" had been in place before the lift.

Workplace Health and Safety Inspectors are currently onsite at Bladin (sic) Point investigating the cause of yesterday's incident, where a heating stack being lifted by two cranes fell.

Preliminary findings show that a risk management plan was in place and an exclusion zone was created before the lift commenced.

No workers where present in the exclusion zone when the incident occurred and there were no injuries as a result of the incident.

The investigation is still ongoing and no further comments will be provided.

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